Effect of Annealing Temperature on the Formation of Silicides and the Surface Morphologies of PtSi Films

The effect of annealing temperature on the formation of the PtSi phase. distribution of silicides and the surface morphologies of silicides films is investigated by XPS. AFM. It is shown that the phase sequences of the films change from Pt-Pt2Si-PtSi-Si to Pt+Pt2Si+PtSi-PtSi-Si or Pt+Pt2Si+PtSi-PtSi-st with an increase of annealing temperature and the reason for the formation of mixed layers is discussed.

SPUTTERING BEHAVIOUR OF IRON SILICIDES UNDER LOWENERGY Ar^+ ION BOMBARDMENT

Surface compositions of Fe3Si, FeSi and FeSi2 under 0.1 to 5ke V Ar+ bombaniment have been investigated by using AES method, and the results indicate that the sudece compositions depend strongly on ion enerpy and sample bulk compositions. While in FeSi and FeSi2 only Fe enrichment in the selvage has been observed, in Fe3Si it is Si enriched when the ion enerpy is higher than 31OeV Competition between preferential sputtering and radiation enhanced segmpation is quoted to eoplain this phenomenon.

Electronic struture of titanium silicides

ＥＬＥＣＴＲＯＮＩＣＳＴＲＵＣＴＵＲＥＯＦＴＩＴＡＮＩＵＭＳＩＬＩＣＩＤＥＳＬｏｎｇ，Ｘｉａｎｇｙｕｎ（ＤｅｐａｒｔｍｅｎｔｏｆＣｈｅｍｉｓｔｒｙ，ＣｅｎｔｒａｌＳｏｕｔｈＵｎｉｖｅｒｓｉｔｙｏｆＴｅｃｈｎｏｌｏｇｙ，Ｃｈａｎｇｓｈａ４１００８３）Ｃ...

Sisyphus effects in hydrogen electrochemistry on metal silicides enabled by silicene subunit edge

Nonmetal elements strictly govern the electrochemical performance of molybdenum compounds.Yet,the exact role played by nonmetals during electrocatalysis remains largely obscure.With intermetallic MoSi_2comprising silicene subunits,we present an unprecedented hydrogen evolution reaction(HER)behavior in aqueous alkaline solution.Under continuous operation,the HER activity of MoSi_2shows a more than one order of magnitude improvement in current density from 1.1 to 21.5 mA cm^(à2)at 0.4 V overpotential.Meanwhile,this activation behavior is highly reversible,such that voltage withdrawal leads to catalyst inactivation but another operation causes reactivation.Thus,the system shows dynamics strikingly analogous to the legendary Sisyphus’labor,which drops and recovers in a stepwise manner repeatedly,but never succeeds in reaching the top of the mountain.Isomorphic WSi_2behaves almost the same as MoSi_2,whereas other metal silicides with silicyne subunits,including CrSi_2and TaSi_2,do not exhibit any anomalous behavior.A thin amorphous shell of MoSi_2is observed after reaction,within which the Si remains partially oxidized while the oxidation state of Mo is basically unchanged.First-principles calculations further reveal that the adsorption of hydroxide ions on silicene subunit edges and the subsequent Si vacancy formation in MoSi_2jointly lead to the anomalous HER kinetics of the adjacent Mo active centers.This work demonstrates that the role of nonmetal varies dramatically with the electronic and crystallographic structures of silicides and that silicene structural subunit may serve as a promoter for boosting HER in alkaline media.

Experimental parameters during optical floating zone crystal growth of rare earth silicides

Optical floating zone（FZ） crystal growth involving growth stability and as-grown crystal perfection is affected by experimental conditions and the specific material. Referring to rare earth silicides, high purity of raw rare earth elements and ambient argon atmosphere are crucial to grow high-quality crystals; the maximum zone height is determined by equating the capillary forces of the surface tension; and asymmetric counter rotation of crystal and feed rod with convex（toward the melt） interfaces are favored to reach single crystals. Influences of several other growth parameters were also discussed in detail in this paper.

Synthesis of Co-silicides and fabrication of microwave power device using MEVVA source implantation

Co synthesis silicides with good properties were prepared using MEVVA ion implantation with flux of 25-125 mA/cm2 to does of 5×1017/cm2. The structure of the silicides was investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM analysis shows that if the ion dose is greater than 2×1017/cm2, a continuous silicide layer will be formed. The sheet resistance of Co silicide decreases with an increase in ion flux and ion dose. The formation of silicides with CoSi and CoSi2 are identified by XRD analysis. After annealing, the sheet resistance decreases further. A continuous silicide layer with a width of 90-133 nm is formed. The optimal implantation condition is that the ion flux and dose are 50 mA/cm2 and 5×1017/cm2, respectively. The optimal annealing temperature and time are 900℃ and 10 s, respectively. The ohmic contact for power microwave transistors is fabricated using Co ion implantation technique for the first time. The emitter contact resistance and noise of the transistors decrease markedly; the microwave property has been improved obviously.

A Review of Metal Silicides for Lithium-Ion Battery Anode Application

Lithium batteries(LIBs) with low capacity graphite anode(~372 mAh g-1) cannot meet the ever-growing demand for new energy electric vehicles and renewable energy storage.It is essential to replace graphite anode with higher capacity anode materials for high-energy density LIBs.Silicon(Si) is well known to be a possible alternative for graphite anode due to its highest capacity(~4200 mAh g-1).Unfortunately,large volume change during lithiation and delithiation has prevented the Si anode from being commercialized.Metal silicides are a promising type of anode materials which can improve cycling stability via the accommodation of volume change by dispersing Si in the metal inactive/active matrix,while maintain greater capacity than graphite.Here,we present a classification of Si alloying with metals in periodic table of elements,review the available literature on metal silicide anodes to outline the progress in improving and understanding the electrochemical performance of various metal silicides,analyze the challenges that remain in using metal silicides,and offer perspectives regarding their future research and development as anode materials for commercial LIBs application.

Improving electrical and thermal properties synchronously via introducing CsPbBr_(3)QDs into higher manganese silicides

Higher manganese silicide(HMS)is a P-type medium temperature thermoelectric(TE)material,which has attracted widespread attention over the past few decades due to its remarkable mechanical properties,excellent chemical and thermal stability,as well as the non-toxicity,abundance and competitive price.The peak power factor(PF)of HMS is as high as~1.50×10^(-3)W m^(-1)K^(-2)because of its intrinsic high electrical conductivity and Seebeck coefficient.However,the thermal conductivity of HMS is also high,resulting in relatively low z T values.Introducing nano-dispersion in the matrix is one of the most effective methods to enhance the TE properties via reducing the lattice thermal conductivity significantly without drastic changes on the other parameters.In this study,Cs Pb BrQDs with uniform size were synthesized and introduced into HMS bulks.The PF(at 823 K)was enhanced to 1.71×10^(-3)W m^(-1)K^(-2),which is improved 14.0%approximately compared with that of pure HMS owing to the combined effect of element doping and energy filtering.The lattice thermal conductivity(at 823 K)decreased from 2.56W mKto 1.99 W mKsynchronously(~22.0%)due to the intensive phonon scattering caused by Cs doping,and the embedding of Pb riched CsPbBr_(3)QDs and Pb QDs.A maximum z T value of 0.57(823 K)is achieved in Cs Pb BrQDs/HMS composites,which is 36.0%higher than that of pure HMS.Predictably,for other TE materials,it is also feasible to improve the TE properties via introducing metastable quantum dots.

Incorporating element doping and quantum dot embedding effects to enhance the thermoelectric properties of higher manganese silicides

Element doping and nano-inclusion embedding are effective approaches to enhance the electrical conductivities and decrease the lattice thermal conductivities of thermoelectric(TE)materials,respectively.However,the intrinsic low electrical thermal conductivities and high electrical properties are severely sacrificed,and the final figure of merit(ZT)is usually restricted.In this study,Ag doping and Pt quantum dot(QD)embedding were synchronously achieved via embedding Ag/Pt alloy QDs into the higher manganese silicides to avoid the conventional single-element doping strategy.The power factor(at 823 K)was enhanced from 1.57×10^(-3) W m^(-1) K^(-2) to 1.82×10^(-3) W m^(-1) K^(-2)(-16%)due to the-18%increase in carrier concentration that was derived from the Ag doping effect.Simultaneously,the lattice thermal conductivity(at 823 K)decreased from 2.65 W m^(-1) K^(-1) e1.92 W m^(-1) K^(-1)(-28%)because of the broadband phonon scattering effect that resulted from the residual Pt QDs inclusions.Synthetically,the optimal ZT value increased by-52%from 0.42 to 0.64 at 823 K.This study demonstrated that incorporating metastable alloy QDs to obtain element doping and nano-inclusion embedding effects is a novel and feasible means to enhance the ZT value of HMS.This method is also possibly applicable to other alloy QD/TE composites.

Sub-nano Layers of Li, Be, and Al on the Si(100) Surface: Electronic Structure and Silicide Formation

Within the framework of the density functional theory and the pseudopotential method,the electronic structure calculations of the“metal-Si(100)”systems with Li,Be and Al as metal coverings of one to four monolayers(ML)thickness,were carried out.Calculations showed that band gaps of 1.02 eV,0.98 eV and 0.5 eV,respectively,appear in the densities of electronic states when the thickness of Li,Be and Al coverings is one ML.These gaps disappear with increasing thickness of the metal layers:first in the Li-Si system(for two ML),then in the Al-Si system(for three ML)and then in the Be-Si system(for four ML).This behavior of the band gap can be explained by the passivation of the substrate surface states and the peculiarities of the electronic structure of the adsorbed metals.In common the results can be interpreted as describing the possibility of the formation of a two-dimensional silicide with semiconducting properties in Li-Si(100),Be-Si(100)and Al-Si(100)systems.

A thin Si nanowire network anode for high volumetric capacity and long-life lithium-ion batteries

Silicon nanowires(Si NWs)have been widely researched as the best alternative to graphite anodes for the next-generation of high-performance lithium-ion batteries(LIBs)owing to their high capacity and low discharge potential.However,growing binder-free Si NW anodes with adequate mass loading and stable capacity is severely limited by the low surface area of planar current collectors(CCs),and is particularly challenging to achieve on standard pure-Cu substrates due to the ubiquitous formation of Li+inactive silicide phases.Here,the growth of densely-interwoven In-seeded Si NWs is facilitated by a thin-film of copper-silicide(CS)network in situ grown on a Cu-foil,allowing for a thin active NW layer(<10μm thick)and high areal loading(≈1.04 mg/cm^(2))binder-free electrode architecture.The electrode exhibits an average Coulombic efficiency(CE)of>99.6%and stable performance for>900 cycles with≈88.7%capacity retention.More significantly,it delivers a volumetric capacity of≈1086.1 m A h/cm^(3)at 5C.The full-cell versus lithium manganese oxide(LMO)cathode delivers a capacity of≈1177.1 m A h/g at 1C with a stable rate capability.This electrode architecture represents significant advances toward the development of binder-free Si NW electrodes for LIB application.

Influence of silicide on fracture behavior of a fully lamellar Ti-46Al-0.5W-0.5Si alloy

The fracture behavior of fully lamellar binaryγ-TiAl alloys is extremely anisotropic with respect to the lamellar orientation.For the fully lamellar Ti-46Al-0.5W-0.5Si alloy,the existence of silicide clusters plays a critical role on the fracture behavior.In the present study,tensile test and three point bending test were performed at room temperature with the loading axis parallel and perpendicular to the lamellar orientation,respectively.To investigate the influence of silicide clusters on the initiation and propagation of cracks,the fracture surface and the cracks adjacent to the fracture zone of the specimens have been analyzed.Results show that the fracture process is related to the morphology and distribution of the silicide clusters.Crack preferentially initiates at and propagates along the interface of silicide andα 2 /γlamellar with the loading axis perpendicular to the length direction of silicide.While the silicide can prevent the propagation of cracks from running across with the crack growth direction perpendicular to the length direction of silicide.

INTERMETALLIC ALLOYS AND COMPOSITES FOR THE 21ST CENTURY: A BASIC ENERGY SCIENCES PROGRAM OF THE U. S. DEPARTMENT OF ENERGY

Progress has been made in intermetallic alloys over the past decade and a half, but intermetallics remain a relatively unexplored class of materials for energy applications. Hence, they offer considerable opportunities both for scientific research on fundamental structural property processing relationships and for technological development. The Department of Energy supports a program of scientific research on intermetallic alloys such as the nickel and iron aluminides and is establishing new research efforts in silicides and Laves phases through the program of the Division of Materials Sciences, of the Office of Basic Energy Sciences. Areas of research include theory and materials simulation, microalloying, high resolution sudies of structure and composition, mechanical properties, point defects and dislocation mechanics, phase transformations, and processing. Research is conducted through programs at the Department of Energy National Laboratories and through grants to academic and industrial researchers.Research results from Division of Materials Sciences programs have provided the basis and transportation. In addition, a cooperative effort between research groups has been established as a project on intermetallic materials under the Center of Excellence in Synthesis and Processing of Advanced Materials.

Oxidation and interdiffusion behavior of a germanium-modified silicide coating on an Nb–Si-based alloy

To investigate the interdiffusion behavior of Ge-modified silicide coatings on an Nb–Si-based alloy substrate,the coating was oxidized at 1250°C for 5,10,20,50,or 100 h.The interfacial diffusion between the(Nb,X)(Si,Ge)_2(X = Ti,Cr,Hf) coating and the Nb–Si based alloy was also examined.The transitional layer is composed of(Ti,Nb)_5(Si,Ge)_4 and a small amount of(Nb,X)_5(Si,Ge)_3.With increasing oxidation time,the thickness of the transitional layer increases because of the diffusion of Si from the outer layer to the substrate,which obeys a parabolic rate law.The parabolic growth rate constant of the transitional layer under oxidation conditions is 2.018 μm×h^(-1/2).Moreover,the interdiffusion coefficients of Si in the transitional layer were determined from the interdiffusion fluxes calculated directly from experimental concentration profiles.

EXPLORATION OF Nb-BASED ADVANCED INTERMETALLIC MATERIAL

Since the late 1980's there have been a number of research efforts aimed at exploring and developing the refractory intermetalllic materials for service at temperatures which compete with the nickel-based superalloys in structural applications. These efforts have documented the physical and mechanical properties of a broad set of compositions. However, only in the last three years have these efforts yielded sufficient experimental results on single selected systems to suggest that damage tolerance, creep resistance and oxidation resistance may be obtained and controlled simultaneously. These findings led to alloy development concepts and approaches which are currently under investigation and are expected to lead to research focused on a smaller set of alloys. An overview of selected alloy development strategies and resulting structural properties is presented herein.

Ternary alloying of Mo_5Si_3 with Zr, Ti, Co and V

The Mo 5Si 3 base alloys with a series of transition metal elements were prepared by arc melting, and were annealed at 1?250?℃ in vacuum for 24?h. Ternary alloying effect was investigated by X ray diffractometry (XRD), optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy(EDS). The EDS results show that Zr, Ti, Co and V have certain solubility in homogenized Mo 5Si 3, which are determined to be 2.20±1.42, 15.94±0.18, 3.33±0.76 and 7.43±0.22 (mole fraction, %), respectively. Microstructural characteristics indicate that all studied alloys have a two phase microstructure, i.e., Mo 5Si 3 matrix and the second phase Mo 37 Zr 20 Si 43 , Mo 66 Si 19 Ti 15 , MoCoSi or (Mo, V) 3Si.

Evolution laws of microstructures and mechanical properties during heat treatments for near-αhigh-temperature titanium alloys

Evolution laws of microstructures,mechanical properties,and fractographs after different solution temperatures were investigated through various analysis methods.With the increasing solution temperatures,contents of the primaryαphase decreased,and contents of transformedβstructures increased.Lamellarαgrains dominated the characteristics of transformedβstructures,and widths of secondaryαlamellas increased monotonously.For as-forged alloy,large silicides with equiaxed and rod-like morphologies,and nano-scale silicides were found.Silicides with large sizes might be(Ti,Zr,Nb)_(5)Si_(3) and(Ti,Zr,Nb)_(6)Si_(3).Rod-like silicides with small sizes precipitated in retainedβphase,exhibiting near 45°angles withα/βboundaries.Retainedβphases in as-heat treated alloys were incontinuous.980STA exhibited an excellent combination of room temperature(RT)and 650°C mechanical properties.Characteristics of fracture surfaces largely depended on the evolutions of microstructures.Meanwhile,silicides promoted the formation of mico-voids.

Atomic diffusion in annealed Cu/SiO_2/Si(100) system prepared by magnetron sputtering

Cu thin films are deposited on p-type Si （100） substrates by magnetron sputtering at room temperature. The interface reaction and atomic diffusion of Cu/SiO2/Si （100） systems are studied by x-ray diffraction （XRD） and Rutherford backscattering spectrometry （RBS）. Some significant results can be obtained. The onset temperature of interdiffusion for Cu/SiO2/Si（100） is 350~℃. With the annealing temperature increasing, the interdiffusion becomes more apparent. The calculated diffusion activation energy is about 0.91 eV. For the Cu/SiO2/Si （100） systems copper silicides are not formed below an annealing temperature of 350~℃. The formation of the copper silicides phase is observed when the annealing temperature arrives at 450~℃.

COMPETING REACTIONS OF EXISTING Ni SILICIDE AND Ni OR Si INDUCED BY THERMAL ANNEALING AND MeV Si ION BEAM MIXING

The competing reactions between existing Ni silicides surrounded by Si and Ni were investigated by thermal annealing and MeV Si ion beam mixing. With high energy irradiation, the energy deposition at both interfaces, Ni/Ni silicide and Ni silicide/Si, is equal. Two MeV He^- RBS and TEM were used to obtain the reacted layer composition and epitaxial orientation, respectively. Also glancing angle Co K_a. X-ray diffraction was utilized to identify phase formation. The main results indicate that the existing silicides preferentially react with Ni layer, and that there are pronounced differences of Ni silicide phase transition between thermal annealing and MeV Si ion beam mixing, even though the mixing was performed in radiation enhanced diffusion regime. The results can be explained in term of the heat of silicide formation and surface energy change.

Defects Interaction on the Mechanical Properties during Transition Formation of (Mo, Cr)<SUB>3</SUB>Si Intermetallic Alloys

Molybdenum silicides alloys with different Mo and Cr additions were produced by the arc cast method. The microstructure revealed mostly single phase structure. Mechanical properties were evaluated in the alloys, showing a decreasing behavior on microhardness. Fracture toughness values were obtained from cracks produced by Vickers indentation technique, showing that ternary alloying did not have a significant effect. Vacancy studies demonstrated that thermal vacancies along the transition line slightly affected the mechanical behavior.